For the first time in three decades, Iran and the United States appear to have established a meaningful diplomatic relationship that disrupts the cycle of escalation towards armed conflict. The interim agreement reached in Geneva on Saturday freezes much of Iran’s nuclear program and exposes the country to extensive inspections by the International Atomic Energy Agency in exchange for modest relief from some economic sanctions. Ultimately, the deal opens the door for a long-term resolution to derail Iran’s nuclear ambitions.

This may be the last, best shot for a diplomatic alternative to a nuclear Iran. Before the deal, Iran could have produced weapons-grade fuel in as little as a month. If Iran upholds its end of the bargain it will lose its stocks of uranium enriched above 5 percent, setting its capabilities back. The chance that Iran will renege on the agreement is real—but so is the risk that US lawmakers will undermine it by passing new sanctions.

On Monday, majority leader Harry Reid said the Senate would consider new economic penalties after the Thanksgiving recess. Leading the process will be Senator Tim Johnson, who chairs the Banking Committee with jurisdiction over sanctions, and Senator Bob Menendez, chair of the Foreign Relations Committee. “They will study this, they will hold hearings if necessary, and if we need work on this, if we need stronger sanctions, I am sure we will do that,” Reid told NPR, adding that the agreement was an “important first step.” Johnson said his committee would hold off from writing new sanctions until after a briefing with Secretary of State John Kerry, Vice President Joe Biden and other administration officials.

Likely, the sanctions the Senate will consider would go into effect only after the interim agreement expires, should the negotiating parties fail to reach a long-term resolution. “I expect that the forthcoming sanctions legislation to be considered by the Senate will provide for a six-month window to reach a final agreement before imposing new sanctions on Iran,” Menendez said in a statement. That seems consistent with what President Obama has said. “If Iran does not fully meet its commitments during this six-month phase, we will turn off the relief and ratchet up the pressure,” he said Sunday.

Sanctions that allow time for diplomatic negotiations to play out are certainly preferable to the immediate punitive measures that lawmakers were calling for last week, and that hawks like Senator Marco Rubio and Representative Kevin McCarthy are still pushing. But that doesn’t mean they’re necessary, or that they won’t hamstring diplomatic negotiations.

First, is Congress’s appetite for a new round of economic punishment really in doubt? Even without new legislation Iranian leaders can easily suppose what Washington’s response will be if a final deal falls through. Meanwhile, most sanctions remain in place and continue to put crippling pressure not only on Iran’s political class but also on its citizens.

On the other hand, new sanctions could ruin the prospects of a long-term accord, even if their practical effects are delayed. There’s a risk that Congress will demand concessions in the short term beyond the scope of the interim agreement, or use new legislation to try to set the terms of a final deal, which could bind negotiators to unobtainable standards. Perceived as belligerence, and a signal that President Obama cannot obtain Congress’s support for dismantling the sanctions regime in a final deal, such moves will likely undermine Iranian President Hassan Rouhani’s ability to sell a long-term agreement to Iran’s political elites.

Rather than doubting that new sanctions will follow a broken agreement, Iran’s leaders fear that the United States won’t allow meaningful economic relief even if Iran scales back its enrichment program. What Rouhani and other moderates need to secure domestic support is evidence that negotiating with the United States will deliver real benefits. Jamal Abdi, policy director for the National Iranian American Council, said that new sanctions “will be seen as an indication that actually the US is going to back out of this or is going to attempt to take advantage of the offer of compromise.”

Abdi explained that the perception that the United States is committed to keeping Iran weak, if not to overthrowing the ayatollahs completely, has been the prevailing narrative among Iran’s political elite for decades. In selling a long-term halt to the nuclear program, Rouhani must prove the opposite: “That Iran can negotiate with the West on equitable terms, that compromise can beget reciprocal compromise and that Iran’s isolation is harmful,” said Abdi. Any new sanctions, on the other hand, would be easily “spun by hardliners eager to find ways to undercut this victory.”

Abdi believes new sanctions could not only derail negotiations with Iran but could cause the existing sanctions regime to collapse. The simple reason is that there’s little left to sanction. Tom Cotton, a Republican Representative from Arkansas, made that clear in May when he proposed going after the relatives of sanctions violators. Besides grandmothers and nephews, policymakers can only increase the pressure on other countries who do business with Iran, and it isn’t clear that they can do so without alienating the governments whose cooperation is needed to enforce punishments already in place. “If there’s a perception that it’s the US that is unwilling to negotiate and is committed to piling on sanctions endlessly, the main rationale for imposing sanctions is going to evaporate,” said Abdi.

Deputy National Security advisor Ben Rhodes summed all of these arguments up on Monday. “I have no doubt that Congress could pass these sanctions very quickly, so we don’t see the need to do it now during the length of this agreement, because, frankly, that could cause divisions within our P5+1 coalition,” Rhodes said, referring to the five countries on the UN Security Council, plus Germany, that signed onto the deal. “It could complicate this diplomacy.”

With little economic slack left to ratchet in on the Iranian regime, that leaves two options for rolling back Iran’s nuclear program, one being the resolution of current negotiations into a long-term deal. The other option is military action. As Joseph Cirincione, president of Ploughshares Fund, told Ezra Klein yesterday, “If you don’t like negotiating with Iran, what you’re really saying is you want to go to war.”

How lawmakers proceed will be an interesting test of the American Isreali Public Affair Committee’s power, which appeared diminished when its aggressive lobbying failed to provoke military action in Syria, which the American public firmly opposed. Now 64 percent of Americans support softening sanctions in exchange for a restriction of Iran’s nuclear program, which the interim agreement achieves. For its part, AIPAC said there would be “no pause, delay or moratorium in our efforts” to ram through additional sanctions. If Congress resists these calls, it could pave the way for a monumental shift in the relationship not only between Iran and the West but also between lawmakers and the Israel lobby.